Problem: On the first 3 exams of his math class, Daniel got an average score of 78. What does he need on the next exam to have an overall average of 81?
Explanation: Let his score on the next exam be $x$ The sum of all of his scores is then $3 \cdot 78 + x$ The same sum must also be equal to $4 \cdot 81$ Solve: $x = 4 \cdot 81 - 3 \cdot 78 = 90$.